Wireless Local Area Network (WLAN) technology standards are established as IEEE (Institute of Electrical and Electronics Engineers) 802.11 standards. IEEE 802.11a and 11b respectively provide maximum data rates of 11 Mbps and 54 Mbps using unlicensed bands of 2.4. GHz and 5 GHz. IEEE 802.11g adopts OFDM (Orthogonal Frequency Divisional Multiplexing) and provides a data rates of 54 Mbps at 2.4 GHz. IEEE 802.11n adopts MIMO-OFDM and provides a data rate of 300 Mbps for 4 spatial streams. IEEE 802.11n supports up to 40 MHz channel bandwidth and provides a maximum data rate of 600 Mbps.
A communication scheme in which operation of an unlicensed user is permitted in a frequency band defined for a licensed user is under discussion. A frequency band which is not used by a licensed user temporarily can be referred to as a whitespace and, particularly, a whitespace in a TV band can be referred to as a TV whitespace (TVWS).
IEEE 802/11af standard for defining operations of unlicensed devices in the TVWS band is currently under development.
TVWS includes VHF (Very High Frequency) bands (54 to 60, 76 to 88 and 174 to 216 MHz) and UHF (Ultra High Frequency) bands (470 to 698 MHz) allocated for TV broadcast and refers to a frequency band permitted for use by an unlicensed device under the condition that communications of licensed devices (devices for TV broadcast, wireless microphones, etc.) operating in the corresponding frequency bands are not hindered.
While operations of all unlicensed devices are permitted in the range of 512 to 608 MHz and 614 to 698 MHz except for some special cases, 54 to 60 MHz, 76 to 88 MHz, 174 to 216 MHz and 470 to 512 MHz are permitted for communication between fixed devices only. A fixed device refers to a device performing transmission at a fixed location only. In the following description, while the TVWS band includes the above-mentioned TVWS, the present invention is not limited thereto.
An unlicensed device that wants to use TV whitespace band needs to provide a licensed device protection function. Accordingly, the unlicensed device must check whether a licensed device occupies the corresponding TV band before starting transmission in the TV whitespace band. That is, the unlicensed device is permitted for use in the whitespace band only when the licensed device is not used in the whitespace band.
To achieve this, the unlicensed device needs to access a geo-location database (GDB) through the Internet or a dedicated network to obtain information about a list of channels available in a corresponding area. The GDB stores and manages information about licensed devices registered therein and information about channels dynamically changed according to geographical positions of the licensed devices and time for which the licensed devices are used. To solve a problem of coexistence of unlicensed devices using the whitespace, a signaling protocol such as a common beacon frame and a spectrum sensing mechanism can be used.
In IEEE 802.11, a TVWS terminal can refer to an unlicensed device operating in the TVWS spectrum using an IEEE 802.11 medium access control (MAC) layer and physical (PHY) layer. In the specification, a station (STA) refers to a TVWS terminal operating in the TVWS spectrum unless otherwise mentioned.
STA needs to provide a function of protecting an incumbent user or a primary user allowed to perform incumbent access including a licensed user (TV user, wireless microphone, etc.). That is, when an incumbent user is using the TVWS, STA has to stop use of the TVWS. Accordingly, STA needs to detect an available channel (channel that is not used by licensed devices) that can be used by unlicensed devices and operate in the available channel.
STA can detect an available channel through spectrum sensing mechanism or by accessing the GDB to determine a TV channel schedule. Energy detection (method of determining that an incumbent user is using TVWS when the intensity of a received signal is higher than a predetermined value) and feature detection (method of determining that an incumbent user is using TVWS when a digital TV preamble is detected) can be used as the spectrum sensing mechanism. In addition, STA needs to access the GDB to acquire GDB information based on the position thereof to check whether a licensed device uses a channel in the position. STA should access the GDB and acquire the information with sufficient frequency to protect the licensed device.
When it is determined that an incumbent user is using a channel immediately adjacent to the currently used channel through the spectrum sensing mechanism or GDB, a terminal (or STA) and a base station (or access point (AP)) can protect the incumbent user by reducing transmit power.